1. Field of the Invention
The present invention relates to an organic light-emitting display device. More specifically, the present invention relates to an organic light-emitting display device which can prevent short circuit between two electrodes driving an organic light-emitting element in a light-emitting element due to the surface roughness of the substrate and provide an improved aperture ratio.
2. Description of the Related Art
As real multi-media era is coming up, flat panel displays (hereinafter, abbreviated to FPDs) used as a man-machine interface are being widely noticed. Conventionally, liquid crystal displays (hereinafter, abbreviated to LCDs) have been used as the FPDs. However, the LCDs have drawbacks such as their slow response and a narrow viewing angle compared with CRT displays.
Recently, as one of the next generation FPDs, display devices using organic light-emitting devices have attracted a great deal of attention. The device using the organic light-emitting device (hereinafter abbreviated to OLED) has superior properties such as spontaneous light emission, wider viewing angle, faster response speed and the like. Conventional structure of OLED is based on a stack of a first electrode preferably formed on glass, an organic light-emitting layer comprising a hole transport layer, a light-emitting layer and an electron transport layer formed on the first electrode, and a second electrode as an upper electrode with a low work function formed on the organic light-emitting layer. Then, by applying a voltage of several volts between the first and the second electrodes, holes and electrons are injected into the hole transport layer and the electron transport layer, respectively. Next, excitons are formed in the light-emitting layer where the holes and the electrons are combined. Finally, light is emitted from the light-emitting layer when the excitons formed return to their ground states. In the case of a so-called bottom emission type using the first electrode which is transparent, the emitted light passes the first electrode and is taken out from the back of the substrate.
Display devices using OLEDs are classified into two groups. One is the OLED of a simple matrix type and another is the OLED of an active matrix type. In the case of the simple matrix type, organic layers, comprising hole transport layers, light-emitting layers and electron transport layers, are formed on the positions where a plurality of anode lines (or anode wirings) and cathode lines (or cathode wirings) cross each other. Each pixel is lit up only for a preset period during one frame. The preset period is the time interval of the one frame divided by the number of the anode lines. The OLED of the simple matrix type has an advantage of its simpler structure.
However, in the case of the simple matrix type, if the number of pixels increases, the preset period should become shorter. Therefore, to achieve a required value for averaged luminance, it is necessary to increase the luminance of the pixel during the preset period. In this case, it causes a problem of a shortened life of the OLED. Furthermore, particularly for a large display panel of OLED, as the OLED is driven by DC, non-uniform potential is applied to each pixel because of the potential drop due to elongated lengths of the anode and cathode lines. As a result, non-uniform luminance is generated over the entire display area. Therefore, qualities in terms of high definition of the pictures and scaling-up of the display size are limited for the simple matrix type.
On the other hand, in the case of an OLED of an active matrix type, each organic light-emitting element comprising each pixel is connected to an element driving circuit (or a pixel driver) comprising 2 to 4 switching elements such as thin film transistor (hereinafter abbreviated to TFT) and capacitors. Also, power lines are formed to supply current to each light-emitting element and thereby all pixels can be simultaneously operated during the period of one frame. Thus, it is not necessary to increase the luminance of the pixels all together and their lives can be elongated. From these reasons, to achieve high definition of pictures and scaling-up (or large display size), the OLED of the active matrix type has an advantage over other types of the OLEDs. Hereinafter, TFTs are used for the switching elements but other active elements can be used instead.
The OLED of the active matrix type which emits light from the back of the substrate is called “bottom emission type”. In the OLED of this type, as the pixel driving elements are installed between the substrate and organic light-emitting elements, as the pixel driving elements obstruct the light emitted from the light-emitting elements and aperture ratio is restricted. Particularly, in the case of large area displays, widths of power lines are widened to reduce the non-uniformity of the luminance of pixels due to potential drop along the power lines, which brings a problem that the aperture ratio of the display becomes smaller. Additionally, if the capacitances of the capacitors are increased to preserve the bias and signal voltages of the transistors to drive the OLED, the area of the capacitor electrodes increases, which accordingly causes a problem that the aperture ratio becomes extremely small.
To solve above problems, efforts to emit the light from the upper electrode using transparent electrode are being made. The OLED with the structure of light emission from the upper electrode is called “a top emission type”. However, in the structure of the top emission type, light-emitting units of OLED are formed on the TFTs, capacitor electrodes or wirings, which causes a problem of a short circuit between the bottom and the upper electrode. It is because the organic light-emitting layer which forms a light-emitting unit and is very thin (about 50-200 nm) can not cover the surface roughness of its underlying layer. This is one of the problems to be sorted out.
To solve this problem, an OLED using a polyimide film as a flattening layer to reduce the surface roughness of its underlying layer was disclosed in JP-A No. 10-189252, which is incorporated herein by reference. FIG. 11 illustrates a sectional view of a structure of a pixel region in the conventional OLED showing the flattening layer formed on its underlying layer. Referring to FIG. 11, a gate insulating film 117 covering a lower capacitor electrode 105 and an upper capacitor electrode 108 are formed on a glass substrate 116. A capacitor which consists of a stack of the lower capacitor electrode 105, the gate insulating film 117 and the upper capacitor electrode 108 is formed.
A 1st insulating interlayer 118 is formed on the stack of the capacitor. On the capacitor, a power line 110 is formed and connected to the upper capacitor electrode 108. Here, a signal line 109 is formed simultaneously. A 2nd insulating interlayer 119 is formed covering the power line 110 and the signal line 109. The flattening layer 136 is formed on the 2nd insulating interlayer 119 and provides a flat surface for a deposition of a lower electrode 115 to form an organic light-emitting element. Reference numeral 120 denotes a 3rd insulating interlayer 120 which prescribes a region of a light-emitting unit 135. Reference numeral 126 denotes a protective layer.
FIG. 12 shows a schematic equivalent circuit diagram of a driving circuit for the organic light-emitting element in the OLED shown in FIG. 11. The same reference numerals shown in FIG. 11 are correspondingly numbered in FIG. 12. Referring to FIG. 12, one pixel consists of an organic light-emitting element 100, a 1st TFT 101 connected to a-scanning line 106 and a signal line 109, a 2nd TFT 102 connected to a power line 110 and a capacitor 104. The scanning line 106 selects the TFT 101 and a signal (i.e. data to be displayed) from the signal line 109 is stored in the capacitor 104. Based on the signal stored in the capacitor 104, the 2nd TFT 102 provides a current from the power line 110 for the organic light-emitting element 100 and then the organic light-emitting element 100 emits light.
Therefore, in the conventional OLED, the surface roughness of the lower capacitor electrode 105 and the upper electrode for the capacitor 108 and steps formed by the power line 110 and the signal line 109 can not be eliminated by the 2nd insulating interlayer 119. Accordingly, a thick flattening layer 136 must be formed to flatten the light-emitting unit 135 consisting of the lower electrode 115, an electron injection layer 124, an electron transport layer 123, a light-emitting layer 122, a hole transport layer 121 and the upper electrode 125. However, to introduce the above flattening layer 136, it requires additional processes such as a spin coating, a baking process, a patterning process using a photolithography, etc, which degrades the reliability of the total process.
In short, based on the prior art, the structure of the OLED of the top emission type is required to ensure a high aperture ratio. Accordingly, the flattening layer 136 with thickness of several μm is necessary to avoid a short circuit between the lower electrode 115 and the upper electrode 125 herewith, where the short circuit is caused by the surface roughness originated from the underlying layers comprising the TFT, wirings, etc.
Thus, a purpose of the present invention is to provide an OLED of the top emission type comprising organic light emitting elements by preventing the previous problems such as the widening of the power lines, the reduction in the aperture ratio caused by the widening of the upper and the lower capacitor electrodes and the short circuit between the upper and the lower electrodes caused by the roughness of the flattening layers.